RFC 6311

Protocol Support for High Availability of IKEv2/IPsec

Internet Engineering Task Force (IETF) R. Singh, Ed.
Request for Comments: 6311 G. Kalyani
Category: Standards Track Cisco
ISSN: 2070-1721 Y. Nir
Check Point
Y. Sheffer
Porticor
D. Zhang
Huawei
July 2011 Protocol Support for High Availability of IKEv2/IPsec
Abstract
The IPsec protocol suite is widely used for business-critical network
traffic. In order to make IPsec deployments highly available, more
scalable, and failure-resistant, they are often implemented as IPsec
High Availability (HA) clusters. However, there are many issues in
IPsec HA clustering, and in particular in Internet Key Exchange
Protocol version 2 (IKEv2) clustering. An earlier document, "IPsec
Cluster Problem Statement", enumerates the issues encountered in the
IKEv2/IPsec HA cluster environment. This document resolves these
issues with the least possible change to the protocol.
This document defines an extension to the IKEv2 protocol to solve the
main issues of "IPsec Cluster Problem Statement" in the commonly
deployed hot standby cluster, and provides implementation advice for
other issues. The main issues solved are the synchronization of
IKEv2 Message ID counters, and of IPsec replay counters.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6311.

Copyright Notice
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document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

there are many issues with the IKEv2/IPsec HA cluster. Sections 3
and 4 below expand on the issues around the IKEv2/IPsec HA cluster
solution, issues which were first described in the problem
statement [6].
In the case of a hot standby cluster implementation of IKEv2/
IPsec-based VPNs, the IKEv2/IPsec session is first established
between the peer and the active member of the cluster. Later, the
active member continuously syncs/updates the IKE/IPsec security
association (SA) state to the standby member of the cluster. This
primary SA state sync-up takes place upon each SA bring-up and/or
rekey. Performing the SA state synchronization/update for every
single IKE and IPsec message is very costly, so normally it is done
periodically. As a result, when the failover event happens, this is
first detected by the standby member and, possibly after a
considerable amount of time, it becomes the active member. During
this failover process, the peer is unaware of the failover event, and
keeps sending IKE requests and IPsec packets to the cluster, as in
fact it is allowed to do because of the IKEv2 windowing feature.
After the newly active member starts, it detects the mismatch in IKE
Message ID values and IPsec replay counters and needs to resolve this
situation. Please see Section 4 for more details of the problem.
This document defines an extension to the IKEv2 protocol to solve the
main issues of IKE Message ID synchronization and IPsec SA replay
counter synchronization, and gives implementation advice to address
other issues. Following is a summary of the solutions provided in
this document:
o IKEv2 Message ID synchronization: This is done by syncing up the
expected send and receive Message ID values with the peer, and
updating the values at the newly active cluster member.
o IPsec replay counter synchronization: This is done by incrementing
the cluster's outgoing SA replay counter values by a "large"
number; in addition, the newly active member requests the peer to
increment the replay counter values it is using for the peer's
outgoing traffic.
Although this document describes the IKEv2 Message ID and IPsec
replay counter synchronization in the context of an IPsec HA cluster,
the solution provided is generic and can be used in other scenarios
where IKEv2 Message ID or IPsec SA replay counter synchronization may
be required.

Implementations differ on the need to synchronize the IKEv2 Message
ID and/or IPsec replay counters. Both of these problems are handled
separately, using a separate notification for each capability. This
provides the flexibility of implementing either or both of these
solutions.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
"SA Counter Synchronization" is the informational exchange defined in
this document to synchronize the IKEv2/IPsec SA counter information
between one member of the cluster and the peer.
Some of the terms listed below are reused from [6] with further
clarification in the context of the current document.
o "Hot Standby Cluster", or "HS Cluster", is a cluster where only
one of the members is active at any one time. This member is also
referred to as the "active" member, whereas the other(s) are
referred to as "standby" members. The Virtual Router Redundancy
Protocol (VRRP) [7] is one method of building such a cluster. The
goal of the hot standby cluster is to create the illusion of a
single virtual gateway to the peer(s).
o "Active Member" is the primary member in the hot standby cluster.
It is responsible for forwarding packets on behalf of the virtual
gateway.
o "Standby Member" is the primary backup member. This member takes
control, i.e., becomes the active member, after the failover
event.
o "Peer" is an IKEv2/IPsec endpoint that maintains an IPsec
connection with the hot standby cluster. The peer identifies the
cluster by the cluster's (single) IP address. If a failover event
occurs, the standby member of the cluster becomes active, and the
peer normally doesn't notice that failover has taken place.
Although we treat the peer as a single entity, it may also be a
cluster.

o "Multiple failover" is the situation where, in a cluster with
three or more members, multiple failover events happen in rapid
succession, e.g., from M1 to M2, and then to M3. It is our goal
that the implementation should be able to handle this situation,
i.e., to handle the new failover event even if it is still
processing the old failover.
o "Simultaneous failover" is the situation where two clusters have
an IPsec connection between them, and failover happens at both
ends at the same time. It is our goal that implementations should
be able to handle simultaneous failover.
o "IPsec replay counter" is the Encapsulating Security Payload (ESP)
Sequence Number or Extended Sequence Number (Section 2.2 of [2]),
or the respective field in the Authentication Header (AH) protocol
(Section 2.5 of [3]).
The generic term "IKEv2/IPsec SA Counters" is used throughout this
document. This term refers to both IKEv2 Message ID counters and
IPsec replay counters. According to the IPsec standards, the IKEv2
Message ID counter is mandatory, and used to ensure reliable delivery
as well as to protect against message replay in IKEv2; the IPsec SA
replay counters are optional, and are used to provide the IPsec anti-
replay feature.

o 3.7. Simultaneous Use of IKE and IPsec SAs by Different Members
* 3.7.1. Outbound SAs Using Counter Modes
o 3.8. Different IP Addresses for IKE and IPsec
o 3.9. Allocation of SPIs
The main problem areas are solved using the protocol extension
defined below, starting with Section 5; additionally, this section
provides implementation advice for other issues in the following
subsections. Implementers should note that these subsections include
a number of new security-critical requirements.
3.1. Large Amount of State
Section 3.2 of the problem statement [6] mentions that a lot of state
needs to be synchronized for a cluster to be transparent. The actual
volume of that data is very much implementation-dependent, and even
for the same implementation, the amounts of data may vary wildly. An
IPsec gateway used for inter-domain VPN with a dozen other gateways,
and having SAs that are rekeyed every 8 hours, will need a lot less
synchronization traffic than a similar gateway used for remote
access, and supporting 10,000 clients. This is because counter
synchronization is proportional to the number of SAs and requires
little data, and the setting up of an SA requires a lot of data.
Additionally, remote access IKE and IPsec SA setup tend to happen at
a particular time of day, so the example gateway with the 10,000
clients may see 30-50 IKE SA setups per second at 9:00 AM. This
would require very heavy synchronization traffic over that short
period of time.
If a large volume of traffic is necessary, it may be advisable to use
a dedicated high-speed network interface for synch traffic. When
packet loss can be made extremely low, it may be advisable to use a
stateless transport such as UDP, to minimize network overhead.
If these methods are insufficient, it may be prudent that for some
SAs the entire state is not synchronized. Instead, only an
indication of the SA's existence is synchronized. This, in
combination with a sticky solution (as described in Section 3.7 of
the problem statement [6]) ensures that the traffic from a particular
peer does not reach a different member before an actual failover
happens. When that happens, the method described in [8] can be used
to quickly force the peer to set up a new SA.

3.2. Multiple Members Using the Same SA
In a load-sharing cluster of the "duplicate" variety (see Section 3.7
of the problem statement [6]), multiple members may need to send
traffic with the same selectors. To actually use the same SA, the
cluster would have to synchronize the replay counter after every
packet, and that would impose unreasonable requirements on the synch
connection.
A far better solution would be to not synchronize the outbound SA,
and create multiple outbound SAs, one for each member. The problem
with this option is that the peer might view these multiple parallel
SAs as redundant, and tear down all but one of them.
Section 2.8 of [4] specifically allows multiple parallel SAs, but the
reason given for this is to have multiple SAs with different Quality
of Service (QoS) attributes. So while this is not a new requirement
of IKEv2 implementations working with QoS, we re-iterate here that
IPsec peers MUST accept the long-term existence of multiple parallel
SAs, even when QoS mechanisms are not in use.
3.3. Avoiding Collisions in SPI Number Allocation
Section 3.9 of the problem statement [6] describes the problem of two
cluster members allocating the same Security Parameter Index (SPI)
number for two different SAs. This behavior would violate
Section 4.4.2.1 of [5]. There are several schemes to allow
implementations to avoid such collisions, such as partitioning the
SPI space, a request-response over the synch channel, and locking
mechanisms. We believe that these are sufficiently robust and
available so that we don't need to make an exception to the rules in
Section 4.4.2.1 of RFC 4301 [5], and we can leave this problem for
the implementations to solve. Cluster members must not generate
multiple inbound SAs with the same SPI.
3.4. Interaction with Counter Modes
For SAs involving counter mode ciphers such as Counter Mode (CTR) [9]
or Galois/Counter Mode (GCM) [10], there is yet another complication.
The initial vector for such modes MUST NOT be repeated, and senders
may use methods such as counters or linear feedback shift registers
(LFSRs) to ensure this property. For an SA shared between multiple
active members (load-sharing cases), implementations MUST ensure that
no initial vector is ever repeated. Similar concerns apply to an SA
failing over from one member to another. See [11] for a discussion
of this problem in another context.

Just as in the SPI collision problem, there are ways to avoid a
collision of initial vectors, and this is left up to implementations.
In the context of load sharing, parallel SAs are a simple solution to
this problem as well.
4. The IKEv2/IPsec SA Counter Synchronization Problem
The IKEv2 protocol [4] states that "An IKE endpoint MUST NOT exceed
the peer's stated window size for transmitted IKE requests".
All IKEv2 messages are required to follow a request-response
paradigm. The initiator of an IKEv2 request MUST retransmit the
request, until it has received a response from the peer. IKEv2
introduces a windowing mechanism that allows multiple requests to be
outstanding at a given point of time, but mandates that the sender's
window should not move until the oldest message it has sent is
acknowledged. Loss of even a single message leads to repeated
retransmissions followed by an IKEv2 SA teardown if the
retransmissions remain unacknowledged.
An IPsec hot standby cluster is required to ensure that in the case
of failover, the standby member becomes active immediately. The
standby member is expected to have the exact value of the Message ID
counter as the active member had before failover. Even assuming the
best effort to update the Message ID values from active to standby
member, the values at the standby member can still be stale due to
the following reasons:
o The standby member is unaware of the last message that was
received and acknowledged by the previously active member, as the
failover event could have happened before the standby member could
be updated.
o The standby member does not have information about on-going
unacknowledged requests sent by the previously active member. As
a result, after the failover event, the newly active member cannot
retransmit those requests.
When a standby member takes over as the active member, it can only
initialize the Message ID values from the previously updated values.
This would make it reject requests from the peer when these values
are stale. Conversely, the standby member may end up reusing a stale
Message ID value, which would cause the peer to drop the request.
Eventually, there is a high probability of the IKEv2 and
corresponding IPsec SAs getting torn down simply because of a
transitory Message ID mismatch and retransmission of requests,
negating the benefits of the high-availability cluster despite the
periodic update between the cluster members.

A similar issue is also observed with IPsec anti-replay counters if
anti-replay protection is enabled, which is commonly the case.
Regardless of how well the ESP and AH SA counters are synchronized
from the active to the standby member, there is a chance that the
standby member would end up with stale counter values. The standby
member would then use those stale counter values when sending IPsec
packets. The peer would drop such packets, since when the anti-
replay protection feature is enabled, duplicate use of counters is
not allowed. Note that IPsec allows the sender to skip some counter
values and continue sending with higher counter values.
We conclude that a mechanism is required to ensure that the standby
member has correct Message ID and IPsec counter values when it
becomes active, so that sessions are not torn down as a result of
mismatched counters.
5. SA Counter Synchronization Solution
This document defines two separate approaches to resolving the issues
of mismatched IKE Message ID values and IPsec counter values.
o In the case of IKE Message ID values, the newly active cluster
member and the peer negotiate a pair of new values so that future
IKE messages will not be dropped.
o For IPsec counter values, the newly active member and the peer
both increment their respective counter values, "skipping forward"
by a large number, to ensure that no IPsec counters are ever
reused.
Although conceptually separate, the two synchronization processes
would typically take place simultaneously.
First, the peer and the active member of the cluster negotiate their
ability to support IKEv2 Message ID synchronization and/or IPsec
replay counter synchronization. This is done by exchanging one or
both of the IKEV2_MESSAGE_ID_SYNC_SUPPORTED and
IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED notifications during the IKE_AUTH
exchange. When negotiating these capabilities, the responder MUST
NOT assert support of a capability unless such support was asserted
by the initiator. Only a capability whose support was asserted by
both parties can be used during the lifetime of the SA. The peer's
capabilities with regard to this extension are part of the IKEv2 SA
state, and thus MUST be shared between the cluster members.
This per-IKE SA information is shared with the other cluster members.

Peer Active Member
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
HDR, SK {IDi, [CERT], [CERTREQ], [IDr], AUTH,
[N(IKEV2_MESSAGE_ID_SYNC_SUPPORTED),]
[N(IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED),]
SAi2, TSi, TSr} ---------->
<-------- HDR, SK {IDr, [CERT+], [CERTREQ+], AUTH,
[N(IKEV2_MESSAGE_ID_SYNC_SUPPORTED),]
[N(IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED),]
SAr2, TSi, TSr}
After a failover event, the standby member MAY use the IKE Message ID
and/or IPsec replay counter synchronization capability when it
becomes the active member, and provided support for the capabilities
used has been negotiated. Following that, the peer MUST respond to
any synchronization message it receives from the newly active cluster
member, subject to the rules noted below.
After the failover event, when the standby member becomes active, it
has to synchronize its SA counters with the peer. There are now four
possible cases:
1. The cluster member wishes to only perform IKE Message ID value
synchronization. In this case, it initiates an Informational
exchange, with Message ID zero and the sole notification
IKEV2_MESSAGE_ID_SYNC.
2. If the newly active member wishes to perform only IPsec replay
counter synchronization, it generates a regular IKEv2
Informational exchange using the current Message ID values, and
containing the IPSEC_REPLAY_COUNTER_SYNC notification.
3. If synchronization of both counters is needed, the cluster member
generates a zero-Message ID message as in case #1, and includes
both notifications in this message.
4. Lastly, the peer may not support this extension. This is known
to the newly active member (because the cluster members must
share this information, as noted earlier). This case is the
existing IKEv2 behavior, and the IKE and IPsec SAs may or may not
survive the failover, depending on the exact state on the peer
and the cluster member.
This figure contains the IKE message exchange used for SA counter
synchronization. The following subsections describe the details of
the sender and receiver processing of each message.

Standby [Newly Active] Member Peer
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
HDR, SK {N(IKEV2_MESSAGE_ID_SYNC),
[N(IPSEC_REPLAY_COUNTER_SYNC)]} -------->
<--------- HDR, SK {N(IKEV2_MESSAGE_ID_SYNC)}
Alternatively, if only IPsec replay counter synchronization is
desired, a normal Informational exchange is used, where the Message
ID is non-zero:
Standby [Newly Active] Member Peer
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
HDR, SK{N(IPSEC_REPLAY_COUNTER_SYNC)} -------->
<--------- HDR
5.1. Processing Rules for IKE Message ID Synchronization
The newly active member sends a request containing two counter
values, one for the member (itself) and another for the peer, as well
as a random nonce. We denote the values M1 and P1. The peer
responds with a message containing two counter values, M2 and P2
(note that the values appear in the opposite order in the
notification's payload). The goal of the rules below is to prevent
an attacker from replaying a synchronization message and thereby
invalidating IKE messages that are currently in process.
o M1 is the next sender's Message ID to be used by the member. M1
MUST be chosen so that it is larger than any value known to have
been used. It is RECOMMENDED to increment the known value at
least by the size of the IKE sender window.
o P1 SHOULD be 1 more than the last Message ID value received from
the peer, but may be any higher value.
o The member SHOULD communicate the sent values to the other cluster
members, so that if a second failover event takes place, the
synchronization message is not replayed. Such a replay would
result in the eventual deletion of the IKE SA (see below).
o The peer MUST silently drop any received synchronization message
if M1 is lower than or equal to the highest value it has seen from
the cluster. This includes any previous received synchronization
messages.

o M2 MUST be at least the higher of the received M1, and one more
than the highest sender value received from the cluster. This
includes any previous received synchronization messages.
o P2 MUST be the higher of the received P1 value, and one more than
the highest sender value used by the peer.
o The request contains a Nonce field. This field MUST be returned
in the response, unchanged. A response MUST be silently dropped
if the received nonce does not match the one that was sent.
o Both the request and the response MUST NOT contain any additional
payloads, other than an optional IPSEC_REPLAY_COUNTER_SYNC
notification in the request.
o The request and the response MUST both be sent with a Message ID
value of zero.
5.2. Processing Rules for IPsec Replay Counter Synchronization
Upon failover, the newly active member MUST increment its own replay
counter (the counter used for outgoing traffic), so as to prevent the
case of its traffic being dropped by the peer as replay. We note
that IPsec allows the replay counter to skip forward by any amount.
The estimate is based on the outgoing IPsec bandwidth and the
frequency of synchronization between cluster members. In those
implementations where it is difficult to estimate this value, the
counter can be incremented by a very large number, e.g., 2**30. In
the latter case, a rekey SHOULD follow shortly afterwards, to ensure
that the counter never wraps around.
Next, the cluster member estimates the number of incoming messages it
might have missed, using similar logic. The member sends out an
IPSEC_REPLAY_COUNTER_SYNC notification, either stand-alone or
together with an IKEV2_MESSAGE_ID_SYNC notification.
If the IPSEC_REPLAY_COUNTER_SYNC is included in the same message as
IKEV2_MESSAGE_ID_SYNC, the peer MUST process the Message ID
notification first (which might cause the entire message to be
dropped as a replay). Then, it MUST increment the replay counters
for all Child SAs associated with the current IKE SA by the amount
requested by the cluster member.
6. IKEv2/IPsec Synchronization Notification Payloads
This section lists the new notification payload types defined by this
extension.

All multi-octet fields representing integers are laid out in big
endian order (also known as "most significant byte first", or
"network byte order").
6.1. The IKEV2_MESSAGE_ID_SYNC_SUPPORTED Notification
This notification payload is included in the IKE_AUTH request/
response to indicate support of the IKEv2 Message ID synchronization
mechanism described in this document.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Protocol ID(=0)| SPI Size (=0) | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size', and
'Notify Message Type' fields are the same as described in Section 3
of [4]. The 'SPI Size' field MUST be set to 0 to indicate that the
SPI is not present in this message. The 'Protocol ID' MUST be set to
0, since the notification is not specific to a particular security
association. The 'Payload Length' field is set to the length in
octets of the entire payload, including the generic payload header.
The 'Notify Message Type' field is set to indicate
IKEV2_MESSAGE_ID_SYNC_SUPPORTED (16420). There is no data associated
with this notification.
6.2. The IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED Notification
This notification payload is included in the IKE_AUTH request/
response to indicate support for the IPsec SA replay counter
synchronization mechanism described in this document.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Protocol ID(=0)| SPI Size (=0) | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size', and
'Notify Message Type' fields are the same as described in Section 3
of [4] . The 'SPI Size' field MUST be set to 0 to indicate that the
SPI is not present in this message. The 'Protocol ID' MUST be set to
0, since the notification is not specific to a particular security

association. The 'Payload Length' field is set to the length in
octets of the entire payload, including the generic payload header.
The 'Notify Message Type' field is set to indicate
IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED (16421). There is no data
associated with this notification.
6.3. The IKEV2_MESSAGE_ID_SYNC Notification
This notification payload type (16422) is defined to synchronize the
IKEv2 Message ID values between the newly active (formerly standby)
cluster member and the peer.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Protocol ID(=0)| SPI Size (=0) | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EXPECTED_SEND_REQ_MESSAGE_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EXPECTED_RECV_REQ_MESSAGE_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
It contains the following data.
o Nonce Data (4 octets): The random nonce data. The data should be
identical in the synchronization request and response.
o EXPECTED_SEND_REQ_MESSAGE_ID (4 octets): This field is used by the
sender of this notification payload to indicate the Message ID it
will use in the next request that it will send to the other
protocol peer.
o EXPECTED_RECV_REQ_MESSAGE_ID (4 octets): This field is used by the
sender of this notification payload to indicate the Message ID it
is expecting in the next request to be received from the other
protocol peer.
6.4. The IPSEC_REPLAY_COUNTER_SYNC Notification
This notification payload type (16423) is defined to synchronize the
IPsec SA replay counters between the newly active (formerly standby)
cluster member and the peer. Since there may be numerous IPsec SAs

established under a single IKE SA, we do not directly synchronize the
value of each one. Instead, a delta value is sent, and all replay
counters for Child SAs of this IKE SA are incremented by the same
value. Note that this solution requires that either all Child SAs
use Extended Sequence Numbers (ESNs) or else that no Child SA uses
ESNs. This notification is only sent by the cluster.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Protocol ID(=0)| SPI Size (=0) | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Incoming IPsec SA delta value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The notification payload contains the following data.
o Incoming IPsec SA delta value (4 or 8 octets): The sender requests
that the peer should increment all the Child SA replay counters
for the sender's incoming (the peer's outgoing) traffic by this
value. The size of this field depends on the ESN bit associated
with the Child SAs: if the ESN bit is 1, the field's size is 8
octets; otherwise, it is 4 octets. We note that this constrains
the Child SAs of each IKE SA to either all have the ESN bit on
or off.
7. Implementation Details
This protocol does not change any of the existing IKEv2 rules
regarding Message ID values.
The standby member can initiate the synchronization of IKEv2 Message
IDs under different circumstances.
o When it receives a problematic IKEv2/IPsec packet, i.e., a packet
outside its expected receive window.
o When it has to send the first IKEv2/IPsec packet after a failover
event.
o When it has just received control from the active member and
wishes to update the values proactively, so that it need not start
this exchange later, when sending or receiving the request.

To clarify the first alternative: the normal IKE behavior of
rejecting out-of-window messages is not changed, but such messages
can still be a valid trigger for the exchange defined in this
document. To avoid denial-of-service (DoS) attacks resulting from
replayed messages, the peer MUST NOT initiate counter synchronization
for any particular IKE SA more than once per failover event.
The standby member can initiate the synchronization of IPsec SA
replay counters:
o If there has been traffic using the IPsec SA in the recent past
and the standby member suspects that its replay counter may be
stale.
Since there can be a large number of sessions at the standby member,
and sending synchronization exchanges for all of them may result in
overload, the standby member can choose to initiate the exchange in a
"lazy" fashion: only when it has to send or expects to receive
traffic from each peer. In general, the standby member is free to
initiate this exchange at its discretion. Implementation
considerations include the ability to survive a certain amount of
traffic loss, and the capacity of a cluster member to initiate
counter synchronization simultaneously with a large number of peers.
8. IKE SA and IPsec SA Message Sequencing
The straightforward definitions of message sequence numbers,
retransmissions, and replay protection in IPsec and IKEv2 are
strained by the failover scenarios described in this document. This
section describes some policy choices that need to be made by
implementations in this setting.
8.1. Handling of Pending IKE Messages
After sending its "receive" counter, the cluster member MUST reject
(silently drop) any incoming IKE messages that are outside its
declared window. A similar rule applies to the peer. Local policies
vary, and strict implementations will reject any incoming IKE message
arriving before Message ID synchronization is complete.
8.2. Handling of Pending IPsec Messages
For IPsec, there is often a trade-off between security and
reliability of the protected protocols. Here again, there is some
leeway for local policy. Some implementations might accept incoming

traffic that is outside the replay window for some time after the
failover event, and until the counters had been synchronized. Strict
implementations will only accept traffic that's inside the "safe"
window.
8.3. IKE SA Inconsistencies
IKEv2 is normally a reliable protocol. As long as an IKE SA is
valid, both peers share a single, consistent view of the IKE SA and
all associated Child SAs. Failover situations as described in this
document may involve forced deletion of IKE messages, resulting in
inconsistencies, such as Child SAs that exist on only one of the
peers. Such SAs might cause an INVALID_SPI to be returned when used
by that peer. Note that Section 1.5 of [4] allows but does not
mandate sending an INVALID_SPI notification in this case.
The IPsecME Working Group discussed at some point a proposed set of
rules for dealing with such situations. However, we believe that
these situations should be rare in practice; as a result, the
"default" behavior of tearing down the entire IKE SA is to be
preferred over the complexity of dealing with a multitude of edge
cases.
9. Step-by-Step Details
This section goes through the sequence of steps of a typical failover
event, looking at a case where the IKEv2 Message ID values are
synchronized.
o The active cluster member and the peer device establish the
session. They both announce the capability to synchronize counter
information by sending the IKEV2_MESSAGE_ID_SYNC_SUPPORTED
notification in the IKE_AUTH exchange.
o Some time later, the active member dies, and a standby member
takes over. The standby member sends its own idea of the IKE
Message IDs (both incoming and outgoing) to the peer in an
Informational message exchange with Message ID zero.
o The peer first authenticates the message. The peer compares the
received values with the values available locally and picks the
higher value. It then updates its Message IDs with the higher
values and also proposes the same values in its response.
o The peer should not wait for any pending responses while
responding with the new Message ID values. For example, if the
window size is 5 and the peer's window is 3-7, and if the peer has
sent requests 3, 4, 5, 6, and 7 and received responses only for 4,

5, 6, and 7 but not for 3, then it should include the value 8 in
its EXPECTED_SEND_REQ_MESSAGE_ID payload and should not wait for a
response to message 3 any more.
o Similarly, the peer should also not wait for pending (incoming)
requests. For example, if the window size is 5 and the peer's
window is 3-7, and if the peer has received requests 4, 5, 6, and
7 but not 3, then it should send the value 8 in the
EXPECTED_RECV_REQ_MESSAGE_ID payload, and should not expect to
receive message 3 any more.
10. Interaction with Other Specifications
The usage scenario of this IKEv2/IPsec SA counter synchronization
solution is that an IKEv2 SA has been established between the active
member of a hot standby cluster and a peer, followed by a failover
event occurring and the standby member becoming active. The solution
further assumes that the IKEv2 SA state was continuously synchronized
between the active and standby members of the cluster before the
failover event.
o Session resumption [12] assumes that a peer (client or initiator)
detects the need to re-establish the session. In IKEv2/IPsec SA
counter synchronization, it is the newly active member (a gateway
or responder) that detects the need to synchronize the SA counter
after the failover event. Also, in a hot standby cluster, the
peer establishes the IKEv2/IPsec session with a single IP address
that represents the whole cluster, so the peer normally does not
detect the event of failover in the cluster unless the standby
member takes too long to become active and the IKEv2 SA times out
by use of the IKEv2 liveness check mechanism. To conclude,
session resumption and SA counter synchronization after failover
are mutually exclusive: they are not expected to be used together,
and both features can coexist within the same implementation
without affecting each other.
o The IKEv2 Redirect mechanism for load balancing [13] can be used
either during the initial stages of SA setup (the IKE_SA_INIT and
IKE_AUTH exchanges) or after session establishment. SA counter
synchronization is only useful after the IKE SA has been
established and a failover event has occurred. So, unlike
Redirect, it is irrelevant during the first two exchanges.
Redirect after the session has been established is mostly useful

for timed or planned shutdown/maintenance. A real failover event
cannot be detected by the active member ahead of time, and so
using Redirect after session establishment is not possible in the
case of failover. So, Redirect and SA counter synchronization
after failover are mutually exclusive, in the sense described
above.
o IKEv2 Failure Detection [8] solves a similar problem where the
peer can rapidly detect that a cluster member has crashed based on
a token. It is unrelated to the current scenario, because the
goal in failover is for the peer not to notice that a failure has
occurred.
11. Security Considerations
Since Message ID synchronization messages need to be sent with
Message ID zero, they are potentially vulnerable to replay attacks.
Because of the semantics of this protocol, these can only be denial-
of-service (DoS) attacks, and we are aware of two variants.
o Replay of Message ID synchronization request: This is countered by
the requirement that the Send counter sent by the cluster member
should always be monotonically increasing, a rule that the peer
enforces by silently dropping messages that contradict it.
o Replay of the Message ID synchronization response: This is
countered by sending the nonce data along with the synchronization
payload. The same nonce data has to be returned in the response.
Thus, the standby member will accept a reply only for the current
request. After it receives a valid response, it MUST NOT process
the same response again and MUST discard any additional responses.
As mentioned in Section 7, triggering counter synchronization by out-
of-window, potentially replayed messages could open a DoS
vulnerability. This risk is mitigated by the solution described in
that section.
12. IANA Considerations
This document introduces four new IKEv2 Notification Message types as
described in Section 6. The new Notify Message Types have been
assigned values as follows.

Appendix A. IKEv2 Message ID Sync Examples
This (non-normative) section presents some examples that illustrate
how the IKEv2 Message ID values are synchronized. We use a tuple
notation, denoting the two counters EXPECTED_SEND_REQ_MESSAGE_ID and
EXPECTED_RECV_REQ_MESSAGE_ID on each protocol party as
(EXPECTED_SEND_REQ_MESSAGE_ID, EXPECTED_RECV_REQ_MESSAGE_ID).
Note that if the IKE message counters are already synchronized (as in
the first example), we expect the numbers to be reversed between the
two sides. If one protocol party intends to send the next request as
4, then the other expects the next received request to be 4.
A.1. Normal Failover -- Example 1
Standby (Newly Active) Member Peer
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Sync Request (0, 5) -------->
Peer has the values (5, 0), so it sends
<------------- (5, 0) as the Sync Response
In this example, the peer has most recently sent an IKE request with
Message ID 4, and has never received a request. So the peer's
expected values for the next pair of messages are (5, 0). These are
the same values as received from the member, and therefore they are
sent as-is.
A.2. Normal Failover -- Example 2
Standby (Newly Active) Member Peer
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Sync Request (2, 3) -------->
Peer has the values (4, 5), so it sends
<------------- (4, 5) as the Sync Response
In this example, the peer has most recently sent an IKE message with
the Message ID 3, and received one with ID 4. So the peer's expected
values for the next pair of messages are (4, 5). These are both
higher than the corresponding values just received from the member
(the order of tuple members is reversed when doing this comparison!),
and therefore they are sent as-is.

A.3. Normal Failover -- Example 3
Standby (Newly Active) Member Peer
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Sync Request (2, 5) -------->
Peer has the values (2, 4), so it sends
<-------------(5, 4) as the Sync Response
In this example, the newly active member expects to send the next IKE
message with ID 2. It sends an expected receive value of 5, which is
higher than the last ID value it has seen from the peer, because it
believes some incoming messages may have been lost. The peer has
last sent a message with ID 1, and received one with ID 3, indicating
that a couple of messages sent by the previously active member had
not been synchronized into the other member. So the peer's next
expected (send, receive) values are (2, 4). The peer replies with
the maximum of the received and the expected value for both send and
receive counters: (max(2, 5), max(4, 2)) = (5, 4).
A.4. Simultaneous Failover
In the case of simultaneous failover, both sides send their
synchronization requests simultaneously. The eventual outcome of
synchronization consists of the higher counter values. This is
demonstrated in the following figure.
Standby (Newly Active) Member Peer
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Sync Request (4,4) ----->
<-------------- Sync Request (5,5)
Sync Response (5,5) ---->
<-------- Sync Response (5,5)